We have discovered that the assembly of hnRNA into ribonucleoprotein particles is blocked during heat shock in Drosophila. We propose that this situation favors the processing of mRNA's whose nuclear maturation is independent of RNP structure. This hypothesis will be tested in a variety of gene systems, with particular emphasis on the nuclear RNP structure of mRNA transcripts of intron-lacking genes. The heat shock block of hnRNP assembly will also be further investigated. We have identified specific proteins bound to small nuclear RNA U1 in Drosophila and have developed a novel approach for studying small nuclear RNP's based on in vitro assembly. We now propose to identify those Drosophila snRNP proteins unique to U1 RNP, to locate the positions of these proteins on U1 RNA by in vitro assembly and nuclease probes, relate U1 RNP structure to the solution secondary structure of Drosphila U1 RNA. Similar studies will be done for Drosophila U4 RNA and RNP. The mRNA's for the major U1 RNP proteins will be cloned and used to study the transcription of these mRNA's and their nucleotide sequence homology with mammalian snRNP protein mRNA's that we are already cloning in separate projects. Small nuclear RNA U1 is thought to be involved in mRNa splicing, but the available evidence is not compelling. We have used psoralen crosslinking to establish that U1 RNA is based-paired with hnRNA in vivo. We will now examine U1 base-pairing in specific Drosophila nuclear RNA transcripts that either contain or lack introns.